Wet de-NOx Process of the Composite Phosphate Rock/H₂O₂ Slurry by Calcination Modification: Key Components and Radical Generation

Wet flue gas denitrification offers a new route for converting industrial nitrogen oxides (NOx) into highly concentrated nitrate wastewater. This study presents a novel H2O2-based advanced oxidation process that utilizes the composite calcined phosphate rock to activate H2O2 for efficient NOx removal. The calcination treatment for composite phosphate rocks was found to enhance de-NOx efficiency and capacity by varying the CaMg(CO3)2 and Ca5(PO4)3OH contents and creating oxygen vacancies. A systematic investigation of the phase structural evolution under alkaline H2O2 conditions identified CaMg(CO3)2 and Ca5(PO4)3OH as the primary active components, with CaMg(CO3)2 preferentially participating in the reaction. Radical quenching experiments and time-resolved electron paramagnetic resonance (EPR) analysis confirmed the predominant roles of superoxide radicals (O2•–) and surface-bound hydroxyl radicals (•OHsurface) in NOx oxidation. A mechanistic pathway was proposed, highlighting the synergistic effects of reactive oxygen species (ROS) and mineral phase transitions on NOx removal. The proposed NOx purification method offers a promising option for nitrogen recovery and NOx pollution control.